Update to latest version of ImageSharp to fix downstream security vulnerability.

Author: HeyJoel

src/Cofoundry.Plugins.Imaging.ImageSharp/Cofoundry.Plugins.Imaging.ImageSharp.csproj

@@ -27,7 +27,7 @@
 
   <ItemGroup>
     <PackageReference Include="Cofoundry.Web" Version="0.12.0" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="3.1.4" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="3.1.5" />
   </ItemGroup>
 
 </Project>